1. Field of the Invention
This invention relates generally to wireless communication systems, and more particularly, to a method of implementing slip compensation for integer frequency offset correction in an OFDM-based wireless communication system.
2. Description of the Prior Art
Coding parameters associated with most wireless communication systems are generally modulated into a transmitted signal that assists with configuration of Customer Premise Equipment (CPE). The CPE modem has to perform integer frequency offset estimation and correction to receive data transmitted by the headend (HE). Orthogonal Frequency Division Modulation (OFDM) is a technique for providing fixed broadband wireless access (BWA) at high data rates to homes and small offices. The HE, using this technique, broadcasts a burst of data using OFDM. As shown in FIG. 1, an OFDM burst is constructed as follows: data symbols, zeros, and pilot/training symbols (total N symbols 110) are assembled in the frequency domain and the inverse FFT (IFFT) of the symbols 125 is output from the IFFT 120 to form a time domain representation of the symbols at 130. A cyclic prefix and postfix are usually added to the resulting time domain samples as shown at block 140. The cyclic prefix is simply the last v samples of the IFFT signal and the cyclic postfix is simply the first L samples of the IFFT signal. The cyclic prefix is appended to the beginning of the N time domain samples and the cyclic postfix is appended to the end of the N time domain samples, resulting in an OFDM burst size of N+v+L samples at 145. FIG. 1 illustrates the construction of an OFDM burst. The OFDM burst can then be upsampled, filtered, upconverted, and transmitted.
The OFDM receiver can have one or two antenna. The following discussion assumes a single receive antenna. Extension to a dual antenna system involves combining the antenna outputs appropriately and the following discussion can be easily be extended to a dual antenna system.
In order to receive the signal from the HE, the CPE modem first has to synchronize to the HE. This implies that the modem has to perform the following initial steps a) check to determine if it is receiving enough signal strength, b) acquire and track the OFDM burst boundaries for proper FFT operation, c) estimate and correct the frequency offset between its local oscillator and that of the HE, d) decode the coding parameter signaling (CPS) parameters transmitted on the training tones, and e) perform channel estimation, data demodulation, decoding (forward error correction, FEC) and medium access controller (MAC) synchronization.
The CPE modem correlates L samples of the received signal after demodulation and analog to digital conversion with L corresponding samples spaced apart by N samples in order to acquire and track the OFDM burst boundaries. The average energy of the 2L samples is then subtracted from the correlation to yield the burst timing metric. In the case of two antennas, the burst timing metric is computed for each of the antennas individually and then combined using any of the standard combining techniques such as equal gain, maximal ratio, etc. Due to the use of a cyclic post-fix, the resulting burst timing cost function (BTCF) has a minimum at the valid start of an OFDM burst boundary. A burst boundary/time tracking algorithm tracks the minimum of the BTCF and configures the hardware and the FFT 230 to start at a valid minimum. Thus, during tracking, the N samples 210 and 220 for the FFT 230 out of the N+v+L sample size burst can be grabbed a few samples earlier or later as compared to the previous burst. This results in what is called a slip such as seen in FIG. 2 at 215.
A slip results in a linear phase difference across the tones (after the FFT 230) between the current and the previous burst. This linear phase is normally taken care of by the channel estimation obtained using the pilot tones. This phase difference should however, be compensated for correct frequency offset estimation which occurs before the channel estimation and CPS decoding steps.
In OFDM-based communication systems, the training tones (i.e. pilot tones) are modulated with CPS data that denote the transmission parameters such as constellation size for data modulation, coding rate for the Reed-Solomon decoder and depth for the interleaver. One technique known to those skilled in the art of wireless communication systems and that has been proposed as a Broadband Wireless Interface Forum (BWIF) standard for broadband wireless interface technology is problematic since it did not consider any timing correction, or slip correction, that might be required when evaluating two consecutive data bursts. Such a timing correction is necessary if and when one data burst slips relative to a previous data burst as a result of variations in a wireless communication channel and/or drift in the CPE clock relative to the HE clock. The present inventors have discovered that known CPS decoding techniques most often fail to recover the foregoing CPS data in the absence of slip correction.
In most wireless communication systems, frequency correction is determined by evaluating a particular cost function. This cost function usually involves the product of several bursts and determining the index where the cost function is a minimum. This minimum location can then be related to the integer frequency offset. This optimization procedure however, often occurs without considering the timing correction required when the bursts slip relative to each other. In view of the foregoing, there is a need in the wireless communication art for a method for achieving slip compensation for integer frequency offset correction in an OFDM-based, wireless communication system.